Abstract Detail

Asynchronous changes in phenology of migrating Broad-tailed Hummingbirds and their early-season nectar resources.

Climate change may lead to temporal mismatches among interacting species because of interspecific variation in phenological responsiveness to increasing temperatures. Species that migrate seasonally across latitudes may be especially prone to altered phenological synchrony with their resources, particularly at the poleward limits of migratory routes, where phenological advancements in the spring are progressing more rapidly than at lower latitudes. Consequently, species migrating from lower latitudes are likely to arrive late to northern breeding grounds relative to the appearance of seasonal resources. To investigate this possibility, we used long-term data sets spanning 27 to 37 years to compare the dates of first arrival of Broad-tailed Hummingbirds (Selasphorus platycercus) to the dates of flowering of nectar plants they visit near the southern and northern limits of their summer breeding grounds. Near the southern limit of its breeding range in Arizona, neither Broad-tailed Hummingbird arrival nor first flowering dates have changed. At a nearby migration stopover site in Arizona, first flowering of a major food plant has advanced, but peak flowering has not. Near the northern limit of its breeding range in Colorado, arrival of Broad-tailed Hummingbirds has advanced less rapidly than has first flowering and peak flowering of two early-season floral resources. These findings are consistent with the expectation that hummingbird arrival to northern breeding grounds is constrained by slower shifts in flowering resource phenology further south along the migration route. The consequence is a shrinking interval of time between arrival of the first hummingbirds and flowering of two important early-season nectar plants at the northern site. If phenological shifts continue into the future, hummingbirds will eventually arrive at northern breeding grounds after flowering begins. Given the reliance of this migratory species on these early-season floral resources, continued change has the potential to reduce hummingbird reproductive success through the mistiming of nesting, reproduction, and peak flowering. It is also likely that pollination and therefore seed production of at least one of these floral resources, Delphinium nuttallianum, will be reduced because of its dependence on hummingbird pollination early in the season. The novel implication of these results is that the breeding range of some species will contract toward lower latitudes under continued climate change.